Any exhibit that engages the minds of children (and adults, but most are already closed for business) with animals is good for animals, and while zoos or taxidermy may not be the most ideal condition for them, it serves a valuable purpose and is a fair sacrifice.
Also, while Knut died at the young age of four (roughly equivalent to a man dying in his early-twenties), he and his twin brother were abandoned by their mother after birth and his brother died a few days afterwards. His four years, a fraction of the usual 25 year lifespan polar bears have, were actually an extension considering that the odds were stacked against him from the beginning. There's a high likelihood that he had some underlying condition that led to him being abandoned in the first place.
